Polycrystalline silicon is used in the semiconductor industry for the manufacture of single crystal silicon ingots. The most prevalent method used to produce single crystal silicon ingots is the Czochralski (CZ) method. Polycrystalline silicon is supplied as rods, chunks, chips, or granules for CZ single crystal ingot production. In the CZ process, a quartz crucible is filled with polycrystalline silicon and loaded into a CZ furnace. After the CZ furnace has been sealed and evacuated, the polycrystalline silicon charge is melted. Once the melt has stabilized, a suspended single crystal silicon seed of a particular crystal orientation is lowered into the molten silicon. The seed is then slowly drawn out of the melt, forming a single crystal silicon ingot. The furnace is turned off, and the ingot is removed from the upper chamber. This ingot is further processed into wafers and then into electronics components such as integrated circuits. In this industry, crystal perfection and purity are critical issues as ever-smaller device line widths demand increased crystalline perfection.
Following the growth of a single crystal silicon ingot, the remaining molten silicon often cracks and destroys the crucible as it cools because of the difference in the coefficient of thermal expansion between quartz and silicon. As the diameter of the single crystal silicon ingot technology increases, the cost of the highly pure quartz crucible increases. For a 300 mm ingot, an exemplar crucible size is 32″ diameter, 20″ tall, holding a charge of 400 kg of polycrystalline silicon. The economics of the process demand a maximum packing density of the polycrystalline silicon charge, maximum use of available quartz crucible space, and crucible re-use if possible.
Maximum packing density in the crucible is achieved through the combination of polycrystalline silicon chunks, chips, rod sections, and granular polycrystalline silicon. Because of the inevitable void space in a packed crucible, the resulting molten silicon volume can be as low as 50% of the available crucible volume. In anticipation of this lost volume, a polycrystalline silicon charge can be mounded above the top of the crucible. With careful mounding of the initial charge the resulting molten silicon volume can only be increased to 70%-80% of the crucible volume, still leaving some of the available volume unused.
The increased performance demanded of silicon based semiconductors has resulted in a reduction in the size and concentration of allowable defects in single crystal silicon ingots. This has required design changes of the CZ furnaces to provide better control of the hot zone. In modem CZ pullers growing ingots having a diameter greater than or equal to 200 mm, it has become common to place a heat shield just above the crucible prior to melting the polycrystalline silicon charge. This heat shield is important for controlling the thermal gradient of the growing ingot. The relationship between thermal gradient and pull speed of the single crystal silicon has been identified as a critical parameter for controlling ingot defects and in particular, crystal originated defects.
Unfortunately, the use of the heat shield significantly limits the amount of polycrystalline silicon that can be mounded above the top of the crucible during the loading process. With the heat shield in place, the level of the molten silicon is on the order of 50% to 70% of the crucible volume. Thus the use of the heat shield, which is critical for reducing defects in larger diameter ingots, can have serious economical impact on the CZ crystal growing process.
There are several techniques for increasing the initial charge size without mounding the polycrystalline silicon above the top of the crucible, and even methods for recharging and reusing a crucible allowing the growth of additional ingots. One way is to introduce granular or chip polycrystalline silicon into the melt through a quartz tube, raising the level of the molten silicon to maximize the available crucible volume. Because of the high surface area to volume ratio of these small pieces, they have the potential to introduce an undesirably high level of impurities into the melt. Another method is to lower one or more polycrystalline rods into the molten silicon before the single crystal growth begins. In this method a ring ditch or keyhole slot is fabricated into the end of one or more polycrystalline feed rods allowing the attachment of a rod holder as shown in Japanese patent publications 2000-178096A, 2000-313690A, or 2001-19587A. The rod holder is suspended from the CZ puller seed holder, and the crystal suspension mechanism is used to lower the polycrystalline silicon rods into the molten silicon. This concept can be used to top off the initial crucible charge, or to recharge the crucible to allow the production of one or more additional single crystal silicon ingots without cooling the crucible to add a new initial charge.
There are disadvantages with current technologies for the use of the suspended polycrystalline silicon rods. In many instances, after the rods are melted into the molten silicon, the rod hanger assembly needs to be retracted out of the furnace and isolated in the upper chamber by closing an isolation valve. The upper chamber is then opened to remove the rod hanger assembly and install the single crystal silicon seed and seed holder. The upper chamber then is evacuated of all oxygen and the valve isolating the furnace is opened. This sequence of events has the potential to introduce impurities into the molten silicon, either through multiple operations of the isolation valve or by introducing additional contaminants when the upper chamber is opened. This also takes time that could be used in growing the single crystal silicon ingot.
The few methods that exist to introduce silicon rods without isolating the upper chamber from the CZ furnace have severe limitations when used with the large volume of charge required for making 200 mm, 300 mm, or larger silicon ingots. One method is to have the single crystal seed incorporated into the rod hanger assembly for the keyhole slotted polycrystalline silicon rod. With the keyhole slot passing all the way across the diameter of the rod, the melted rod will fall away from the holder, leaving the single crystal seed exposed to begin the crystal pulling process. In this manner, one polycrystalline silicon rod can be added to the molten silicon without cycling the furnace isolation valve.
The charge size for a 300 mm CZ crystal pulling system is on the order of 400 kg, and to make up, for example, 30% of the crucible volume after the initial charge melts requires 120 kg of additional charge. Polycrystalline silicon rods produced by the Siemens method have internal stresses that increase with the diameter of the rod. If these internal stresses are too high, the rod may break apart when subjected to the thermal gradients in the CZ furnace, causing damage to the furnace. A suitably sized polycrystalline silicon rod for lowering into the CZ furnace is 150 mm in diameter and 900 mm long. Such a rod has a low risk of breaking during the process. But a polycrystalline silicon rod of this size has a mass of 37 kg, and thus provides less than a third of the material required to top off a large crucible.
Methods and apparatuses of the type described herein are useful in dealing with these problems. They can provide for the addition of a sufficient mass of polycrystalline rod material to maximize the use of the crucible volume in the production of large diameter CZ silicon ingots. Multiple silicon rods can be added into the furnace and the subsequent production of a single crystal silicon ingot can occur without operating the isolation valve or opening the upper chamber. This enables CZ silicon ingot manufacturers to increase their yields and product purity.